Historically, sheet metal sections have been used as frames for motor vehicles. A relatively recent development in motor vehicles is the use of tubular frames instead of the traditional sheet metal frames, to reduce weight and expense. Thus, it is known in the automotive industry to provide a frame for vehicles upon which various parts or body panels can be mounted. Such frames can be formed from a number of individual straight tubular (pipe) members that are connected to each other to form tubular sections. These tubular sections can form a prehydroformable birdcage then be hydroformed into the final shape.
Tubular members are advantageous because of their ready availability and good weight to strength ratio. As a result, the use of such tubular members is economically advantageous. The final shapes of these members can vary greatly from their original tubular configuration and are often, in fact, substantially rectangular. While these tubular members have been, in the past, often stamped into shape, a particularly advantageous method of forming these tubular sections into their final shape is through a hydroforming process.
Known hydroforming processes are identified in, for example, U.S. Pat. Nos. 6,216,509 B1 and 6,183,013 B1, in addition to published international application No. WO 99/20516. These patents are expressly incorporated as if set forth in their entirety herein.
WO 99/20516 discloses a hydroformed space frame and a method of manufacturing the same. Individual side rails are formed from a straight tubular blank which is bent into an S-shape and then hydroformed into the correct shape, with varying cross-sectional diameters over the length of the frame section. Individual sections are butt-welded together before the hydroforming operation to form larger sections, which are joined subsequent to the hydroforming operation.
U.S. Pat. No. 6,183,013 B1 discloses a hydroformed side rail for a vehicle frame and a method of its manufacture. According to this patent, the side rails for the vehicle frame have varying wall thicknesses and cross-sectional areas along the length of the elements. Individual portions of the rail are hydroformed and, subsequent to hydroforming, are combined into the side rail.
U.S. Pat. No. 6,216,509 B1 discloses a hydroformed tubular member and a method of hydroforming tubular members. Tubular members are formed, as taught in this patent, into end products having various circumferences, diameters and gages by varying the size of the blanks that the tube is hydroformed into.
In general, a hydroforming process forms a tubular assembly into a desired final shape by placing the tubular assembly into a die of the desired final shape and introducing a pressurized fluid into the interior of the tubular blank. The pressurized fluid shapes the tubular assembly to fit the die so as to form the final structure.
It can be readily seen from the above that it is important to the hydroforming operation that the connections between tube parts be secure and resistant to leakage, and preferably leak-tight. These connections must be able to withstand the extreme stresses applied during the hydroforming process, and yet be able to retain their structural strength in the final, formed shape. This is especially true in automotive applications, wherein the hydroformed part is typically used as a structural frame member.
Presently, the parts to be hydroformed are generally welded together, e.g. by seam welding, before being subjected to the hydroforming process. Great care must be taken to insure that the weld is of sufficient strength to withstand the hydroforming stresses.
In view of the above, it would be advantageous to develop a hydroforming process for parts to be bonded together in a reliable, economical manner, the bond being of sufficient strength to withstand the hydroforming process, substantially without leakage, and then retaining sufficient bond strength, following the hydroforming process, to be used as a structural member in, for example, an automotive application and other such applications.